The present invention relates generally to an optical ring resonator type gyro commonly referred to as an optical passive ring resonator gyro (OPRG) wherein coherent light is beams frequency shifted and introduced into a ring-shaped optical path so as to propagate therethrough in opposite directions to each other as clockwise and counterclockwise light beams respectively, and after the light beams have propagated through the optical path in opposite directions a plurality of times so that they undergo multiple Interference with each other, a portion of each light beam is taken out of the optical path to detect the light intensity and the thus detected outputs are negatively fed back to control the frequency shifts of the clockwise and counterclockwise light beams, thereby detecting an angular rate or velocity applied to the ring-shaped optical path.
A description will be given first of an optical ring resonator. In the optical ring resonator, when the effective optical path length corresponding to the entire length of the ring is an integral multiple of the wavelength of light, the multiple interference light satisfies conditions for resonance and hence goes bright and has large energy. As shown in FIG. 1, coherent light is introduced into a ring resonator 11 via an optical directional coupler 13 from a light source 12 to propagate through the resonator 11 (clockwise, in this example), a portion of the light is taken out of the ring resonator 11 via an optical directional coupler 14 and the intensity of such light is detected by a photodetector 15. The light intensity I thus detected has a characteristic such as indicated by the curve 16 in FIG. 1A with respect to the frequency of light from the light source 12. At the frequency which satisfies the conditions for resonance the detected light intensity increases abruptly, and at this time, the interference light goes bright.
On the other hand, a portion of the light beam is taken out of the ring resonator 11 also via the optical directional coupler 13 through which the light beam is introduced into the ring resonator 11. The light beam thus taken out interferes with the incident light beam and the intensity of the interference light, detected by a photodetector 17 has a frequency characteristic such indicated by the curve 18 in FIG. 1B. At the frequency which satisfies the condition for resonance the detected light intensity decreases sharply and the interference light goes dark. The characteristics 16 and 18 obtainable with the photodetectors 15 and 17 are sometimes called the transmission characteristic and reflection characteristic of the optical ring resonator 11, respectively, and either of them, can be used to form the optical ring resonator gyro. Since the use of optical couplers in combination with the optical ring resonator usually deteriorates its characteristic, the reflection characteristic is utilized in many cases.
Now, the outline will be given of a conventional optical ring resonator gyro which utilizes the reflection characteristic. The reflection characteristic 18 is given by the following expression: EQU I=I.sub.0 .nu.{1-.alpha./[1+.beta. sin.sup.2 (f.tau./2)]}
where I is the output light intensity, I.sub.O the incident light intensity, .nu. an optical coupler loss, .alpha. and .beta. constants dependent on the optical system used, f the optical frequency, and .tau. the time for light to make a round of the ring of the optical ring resonator.
The principle of the optical ring resonator gyro is based on the Sagnac effect. The Sagnac effect is an effect wherein when a ring-shaped optical path surrounding a limited closed area performs an angular velocity motion about a normal to the closed area, a difference occurs in the effective propagation time between both light beams propagating in the ring-shaped optical path in opposite directions.
The effective propagation time difference .DELTA..tau. is given by the following expression: EQU .DELTA..tau.=(4S/C.sup.2).OMEGA.
where S is the closed area, C the velocity of light and .OMEGA. the angular velocity. When the ring-shaped optical path constitutes a resonator, the condition for resonance of light, given by the following expression, also differs between the two light beams owing to the Sagnac effect: EQU f.sub.r =M/.tau.
where f.sub.r is the light resonance frequency, .tau. the time for light to make a round of the ring-shaped optical path and m an integer. And the resonance frequency difference .DELTA.f.sub.r between the two light beams is as follows: EQU .DELTA.f.sub.r =(4S.lambda.L).OMEGA.
where .lambda. is the wavelength of light and L the length of the ring-shaped optical path. By introducing coherent light into the optical ring resonator in opposite directions and detecting the resonance frequency difference between the two light beams at the same time, the angular rate or velocity applied to the optical ring resonator can be determined. The angular velocity sensor based on this principle is commonly referred to as an optical passive ring resonator gyro.
FIG. 2 shows in block form the construction of such a conventional optical ring resonator gyro. Light emitted from the light source 12 is split by an optical directional coupler 19 into two light beams, which are frequency modulated by optical frequency shifters 21 and 22, respectively, and the two frequency-modulated light beams are introduced by the optical directional coupler 13, as clockwise and counterclockwise light beams, into the ring-shaped optical path 11 formed by an optical waveguide as of glass or optical crystal. The oscillation output of an oscillator 23 is usually a sine-wave and the sine-wave signal is applied as a control signal to variable frequency oscillators (VCO's) 26 and 27 via adders 24 and 25, respectively, by which the oscillation frequencies of the variable frequency oscillators 26 and 27 are varied sinusoidally. The oscillation outputs from the variable frequency oscillators 26 and 27 are applied to the optical frequency shifters 21 and 22 to drive them.
A portion of both light beams, which have propagated in the ring-shaped optical path 11 a plurality of times and have undergone multiple interference with each other, is led out of the optical path 11 via the optical directional coupler 13, wherein the light beams thus led out of the optical path 11 interfere with light beams from the optical frequency shifters 21 and 22 for incidence to the optical path 11, respectively. The resulting interference light beams are provided via optical directional couplers 28 and 29 to photodetectors 31 and 32 for conversion to electrical signals. The outputs from the photodetectors 31 and 32 are applied to lock-in amplifiers 33 and 34, wherein they are synchronously detected by the output from the oscillator 23. The detected outputs are applied to the adders 24 and 25, wherein they are each added with the output from the oscillator 23, and the added outputs are provided as the control signals to the VCO's 26 and 27.
In the case where the frequency of the output light from the one optical frequency shifter 21 is frequency modulated by a sine wave from the oscillator 23 which has a period T as indicated by the curve 35 in FIG. 3A and the center frequency is made to agree with the frequency at the bottom of one fall or drop in the reflection characteristic 18 shown in FIG. 1B, the intensity of the signal that is provided from the optical directional coupler 28 to the photodetector 31 corresponding to that one fall or drop characteristic 36 varies at a frequency twice higher than the output frequency of the oscillator 23, as indicated by the curve 37. In consequence, the output from the lock-in amplifier 33 and hence the output from the synchronous detector is reduced to zero.
When an angular rate is applied to the ring-shaped optical path 11 about its axis, the resonance frequency of the ring-shaped optical path 11 is shifted by the Sagnac effect; for example, as shown in FIG. 3B, the frequency of the reflection characteristic which takes the minimum value of the fall-in characteristic 36 deviates from the center of the frequency of the output light of the optical frequency shifter 21. As a result, the signal intensity, which is provided at the output of the photodetector 31, includes the main component of the output frequency of the oscillator 23 as indicated by the curve 37. Thus the output of the lock-in amplifier 33, that is, the synchronously detected output corresponds to the input angular rate. The output from the lock-in amplifier 33 is negatively fed back to the VCO 26 via the adder 24, by which the center frequency for the optical frequency modulation by the optical frequency shifter 21 is shifted to remove the deviation from the frequency of the reflection characteristic which takes the minimum value of the fall-in characteristic 36.
Also for the light obtained by introducing the output light from the other optical frequency shifter 22 into the ring-shaped optical path 11, the same operation as described above is carried out by the photodetector 32, the lock-in amplifier 34, the adder 25 and the VCO 27. Hence the clockwise and counterclockwise light beams in the ring-shaped optical path 11 are always controlled so that the centers of their frequencies resonate in the optical path 11. The frequency difference between the outputs from the VCO's 26 and 27 is detected, as the frequency difference between the two light beams, by a double balanced mixer 38, and this difference frequency is a detected quantity indicative of the angular rate input into the ring-shaped optical path 11.
The optical frequency shifters 21 and 22 are typically acoustooptic devices which utilize the Bragg diffraction.
Incidentally, the above-described optical passive ring resonator gyro calls for two optical frequency shifters 21 and 22. The acoustooptic devices, which are actually used as optical frequency shifters, are bulky, heavy, high in light loss, large in power consumption, unstable in operation and radiate high frequency magnetic fields, which constitute a leading factor in the generation of noise. Moreover, since the modulated light beams must be transmitted through such relatively large acoustooptic devices as three-dimensional media, much difficulty is involved in the alignment of optical axes and other packaging work. Besides, electrical circuits for driving use generate much heat.
Optical frequency shift means free from the above-said shortcomings is what is called direct modulation in the case of using a semiconductor laser as the light source. Direct modulation utilizes a characteristic that the laser oscillation frequency deviates linearly in the operational region in accordance with the laser drive current. Since the optical passive ring resonator gyro requires independent optical frequency control means for each of the clockwise and counterclockwise light beams as referred to above, however, even if direct modulation is applied to the light source which is used for both light beams in common thereto, at least one external optical frequency shifter is needed in the case of employing the conventional method.